Warming Shrinks Arctic Sea Ice to Lowest Level On Record

In a Melting Trend, Less Arctic Ice to Go Around

The New York Times, Sept. 29, 2005

By Andrew C. Revkin

The floating cap of sea ice on the Arctic Ocean shrank this summer to what is probably its smallest size in at least a century of record keeping, continuing a trend toward less summer ice, a team of climate experts reported yesterday.

That shift is hard to explain without attributing it in part to human-caused global warming, the team's members and other experts on the region said.

The change also appears to be headed toward becoming self-sustaining: the increased open water absorbs solar energy that would otherwise be reflected back into space by bright white ice, said Ted A. Scambos, a scientist at the National Snow and Ice Data Center in Boulder, Colo., which compiled the data along with the National Aeronautics and Space Administration.

"Feedbacks in the system are starting to take hold," Dr. Scambos said. The data was released on the center's Web site, www.nsidc.org

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The findings are consistent with recent computer simulations showing that a buildup of smokestack and tailpipe emissions of greenhouse gases could lead to a profoundly transformed Arctic later this century, when much of the once ice-locked ocean would routinely become open water in summers.

Expanding areas of open water in the summer could be a boon to whales and cod stocks, and the ice retreat could create summertime shipping shortcuts between the Atlantic and the Pacific.

But a host of troubles lie ahead as well. One of the most important consequences of Arctic warming will be increased flows of meltwater and icebergs from glaciers and ice sheets, and thus an accelerated rise in sea levels, threatening coastal areas. The loss of sea ice could also hurt both polar bears and Eskimo seal hunters.

The Arctic ice cap always grows in the winter and shrinks in the summer. The average minimum area from 1979, when precise satellite mapping began, until 2000 was 2.69 million square miles, similar in size to the contiguous area of the United States. The new summer low, measured on Sept. 19, was 20 percent below that.

Before 1979, scientists estimated the size of the ice cap based on reports from ships and airplanes.

The difference between the average ice area and the area that persisted this summer was about 500,000 square miles, an area about twice the size of Texas, the scientists said.

This summer was the fourth in a row with the ice cap areas sharply below the long-term average, said Mark C. Serreze, a senior scientist at the snow and ice center and a professor at the University of Colorado, Boulder.

Dr. Scambos said the consecutive reductions in the ice cap "make it pretty certain a long-term decline is under way."

A natural cycle in the polar atmosphere called the Arctic oscillation, which contributed to the reduction in Arctic ice in the past, did not appear to be a factor in the past several years, Dr. Serreze said.

He said the role of accumulating greenhouse gas emissions had become increasingly apparent with rising air and sea temperatures. Still, many scientists say it is not yet possible to determine what portion of Arctic change is being caused by rising levels of carbon dioxide and other emissions from human sources and how much is just climate's usual wiggles.

Dr. Serreze and other scientists said that more variability could lie ahead and that the area of sea ice could actually increase some years. But the scientists have found few hints that other factors, like more Arctic cloudiness in a warming world, will reverse the trend.

"With all that dark open water, you start to see an increase in Arctic Ocean heat storage," Dr. Serreze said. "Come autumn and winter that makes it a lot harder to grow ice, and the next spring you're left with less and thinner ice. And it's easier to lose even more the next year."

The result, he said, is that the Arctic is "becoming a profoundly different place than we grew up thinking about."

Other experts on Arctic ice and climate disagreed on details. For example, Ignatius G. Rigor at the University of Washington said the change was probably linked to a mix of factors, including influences of the atmospheric cycle.

But he agreed with Dr. Serreze that the influence from greenhouse gases had to be involved.

"The global warming idea has to be a good part of the story," Dr. Rigor said. "I think we have a different climate state in the Arctic now. All of these feedbacks are starting to kick in and really snuffing the ice out by the end of summer."

Other experts expressed some caution. Claire L. Parkinson, a sea ice expert at NASA's Goddard Space Flight Center in Greenbelt, Md., said a host of changes in the Arctic - including rising temperatures, melting permafrost and shrinking sea ice - were consistent with human-caused warming. But she emphasized that the complicated system was still far from completely understood.

William L. Chapman, a sea ice researcher at the University of Illinois Urbana-Champaign, said it was important to keep in mind that the size of the ice cap could vary tremendously, in part because of changes in wind patterns, which can cause the ice to heap up against one Arctic shore or drift away from another.